1. Field of the Invention
This invention relates to a car navigation system which determines the present position of the car from a radio wave signal from a GPS satellite, and leads the car to a predetermined destination, and more particularly to a car navigation system which is suitably mounted on a car.
2. Related Art
In general, a car navigation system comprises: an antenna for receiving a radio wave signal from a GPS satellite; a controller which performs various operations to determine the present position of a car according to the signal received by the antenna and to lead the car to a predetermined destination; and an image display (or a monitor such as a liquid crystal display) which is controlled by the controller to display a map and the present position of the car.
A car navigation system of this type has a route setting function to automatically setting a route to a destination. Where this route setting function is used, a route to a destination is indicated on a map displayed on the monitor, and the present position and the direction in which the car should move are indicated with marks such as arrow marks, and when the car comes near a branching guide point such as a crossing, a simple map is displayed on a large scale to show the area around the branching guide point in detail, and in addition, a vocal guide announcement is made--for instance "Car is coming near the guide point", "Turn right at the next corner", "The next relay point is on the left side".
However, the conventional car navigation system suffers from the following problems:
(1) In general, the antenna is mounted on the roof of the car, and the controller and the monitor (display) are mounted on the arm rest beside the driver's seat or on the instrument panel so that they may not come in the front field of vision of the driver operating the car. The monitor is relatively small in size. Hence, during the day time when it is light outside the car, it is rather difficult for the driver to see a picture displayed on the monitor (especially in the case where a liquid crystal display is employed as the monitor). Sometimes, the driver cannot read the contents displayed, and even a fellow passenger cannot read the contents.
(2) In some of the navigation systems, a TV tuner is built in the controller or the monitor. When television is watched by using the monitor, it goes without saying that no navigation guide is displayed on the monitor.
(3) Reading the picture (such as the position of the car, and the direction of traveling) displayed, especially to watch the monitor for a while which is difficult to see, may result in an accident because, in this case, the driver takes his eyes off the road because of the position of the monitor. Hence, even when the car comes near, for instance, a crossing where the car is to be turned right or left, the driver operating the car cannot read the indication on the monitor except when a fellow passenger is in the car. This is inconvenient for the driver.
(4) In the case when the driver and a fellow passenger or passengers are talking to each other, or when an audio set is in operation, it is difficult for the driver to hear the vocal guide announcement. In addition, the vocal guide announcement is made only two or three times. Hence, if the driver misses it, then there is no other way for the driver to be alerted. That is, the vocal guide announcement is lower in certainty and in reliability than the visual recognition through display on a monitor.